Influences of the propyl group on the van der Waals structures of 4-propylaniline complexes with one and two argon atoms studied by electronic and cationic spectroscopy

4-propylaniline complexes with one and two argon atoms formed in the molecular beam were studied in the first excited electronic state, S₁, using resonance enhanced two-photon ionization spectroscopy and in the cation ground state, D₀, using mass analyzed threshold ionization spectroscopy. The combination of electronic and cationic spectra of the clusters allows two conformations to be identified in both aniline-Ar₁ and aniline-Ar₂, which are assigned to either the gauche configuration or anti-configuration of 4-propylaniline. The gauche isomer exhibits complex bands shifted 29 cm⁻¹ and 89 cm⁻¹ from the S₁ origin bands and 83 cm⁻¹ and 148 cm⁻¹ from the ionization potential assigned to the Ar₁ and Ar₂ complexes, respectively. For the anti-rotamer, the corresponding shifts actually become nearly additive, 53 cm⁻¹ and 109 cm⁻¹ for the S₁ origin bands, and 61 cm⁻¹ and 125 cm⁻¹ for the ionization potentials. Ab initio calculations provide insights into the influences of the propyl and amino groups on the positions of the argon atoms within the clusters. In addition, the binding energy of one argon with the gauche isomer of 4-propylaniline has been measured to be 550 ± 5 cm⁻¹ in the D₀ state, 496 ± 5 cm⁻¹ in the S₁ state, and 467 ± 5 cm⁻¹ in the neutral ground state, S₀.